1. Field of the Invention
The present invention is directed to a defibrillator having a charging capacitance that can be switched at both its sides for connection to a charging circuit and that can be connected via a switch means to at least two electrodes arranged in the region of the heart for defibrillating a heart.
2. Description of the Prior Art
A defibrillator of the type described above is disclosed in U.S. Pat. No. 4,800,883 is intended for implantation in the body of a patient. This known defibrillator contains a charging capacitance composed of two capacitors that are connected to a charging circuit for charging to a given voltage. The two capacitors are also connected to two electrodes placed at the heart of the patient, via a switch means composed of four switches arranged in bridge circuitry. For defibrillation of the heart, the charging capacitance is first charged to the given voltage and is subsequently connected to the electrodes at the heart via the switch means, so that the charging capacitance discharges with a discharge current via the heart tissue. The four switches of the switch means are alternately opened and closed in pairs, so that the discharge current through the heart tissue is subdivided into a plurality of successive sub-currents having alternating direction of the current.
A circuit is described in European Application 0 060 404 which limits the current through the paddles of an external defibrillator. This circuit, however, makes use of inductances for the current limitations which are too heavy and large for use in an implantable device.
The current through the heart tissue that produces defibrillation is dependent on the charging voltage of the charging capacitance and on the electrical resistance of the heart tissue between the electrodes. The current has its highest value at the beginning of the discharge of the charging capacitance and then exponentially decays. In order to obtain an effective defibrillation of the heart, the current through the heart tissue must exceed a specific minimum value over a specific duration. For this reason, the charging voltage for the charging capacitance is selected such that the current is adequately high at the beginning of the discharge event so that it does not drop below the minimum value until the expiration of the specific duration. The part of the current exceeding the minimum value is not only useless in view of an effective defibrillation but, moreover may cause damage to the heart tissue as a consequence of the high initial value. A further problem is that shorts can occur between the electrodes due to a fault or due to damage in the leads of the electrodes or given dislocation of the electrodes, so that the short-circuit current arising when triggering the discharge of the charging capacitor can cause damage to the patient and to the defibrillator. Such shorts can also occur during manufacture and during later handling of the defibrillator in conjunction with the implantation thereof, so that there is a risk to the person handling the defibrillator.